How should I insulate my basement?

Question

My basement is CMU blocks and concrete floor.

I had water issues, so I waterproofed it with an interior french drain, drainboard at the edge and a sump pump. This wouldn't take care of dampness so I added a dehumidifier.

I plan to finish the basement and here's my understanding of how I should go about it:

I think I'll avoid vapor barrier so I don't get mold problems. It gets quite humid around here. I'm in Washington DC on the east coast and if I interpret correctly the following: http://www.house-energy.com/Basements/Interior.htm, I shouldn't use vapor barrier. I have a few minor cracks in the wall maybe I should fill those may be not.

Rigid extruded polystyrene installed directly on the wall. Two inch R10 perhaps tucked into the drainboard of the french drain. This way any moisture will go down into the drain.

Leave some space (inch or two) then build the wall from the ground so it's completely detached from the wall.

Not sure what I'd do with the floor; maybe would insulate it maybe not. Height is only 7ft so any inch of height is precious.

Here's an article about this in BuildingScience.com: http://www.buildingscience.com/documents/reports/rr-0309-renovating-your-basment

This is all theoretical, I've never done anything like this, so I'm looking for feedback. Also, would I need insulation on the ceiling? I like the idea of spraying fire-rated foam, is it an overkill? Guess, I'd need soundproofing if nothing else.

Edit:

Since I'm starting with the walls, here are my first-priority questions:

• Fill cracks or not?

• Vapor barrier or not?

• 2 inch rigid foam or just 1 inch and then more insulation next to the dry wall? (I'm thinking all insulation better be away from the dry wall to avoid any condense around it)

• Do I tuck it into the drainboard?

• Do I glue the insulation or is there another method of attaching?

• Use spray foam to insulate the parts of the wall between the studs? Or is the rigid foam going to be fine? (I'm thinking the spray would fill all gaps, therefore'd be better)

Answer 1

Why would you need insulation on the ceiling in a basement? Since the ceiling is touching a heated space itself, there is no need if you will then be using the basement too. Insulation there should be done only for soundproofing, unless you have radiant underfloor heat installed, in which case, the insulation should have been done already as part of the installation. So insulation really only offers soundproofing value. So the question is, how much noise will be generated there?

Counterbalance that with the rationale for leaving this space accessible. I've often enough needed access to the underfloor regions of my home, for wiring changes, plumbing changes, repairs, etc. So any insulation may be better done as fiberglass batts anyway.

As far as the walls go, filling the cracks is a reasonable idea, IF they are stable. If the walls have moved due to frost heaves, etc. and might continue to move, I would ensure they are stabilized before you cover the wall over. A buckling foundation wall is not something good to have under your house.

Answer 2

I am in a similar situation, and I believe you are correct.

In some cases a vapor barrier helps, but in some cases it does not. I installed rigid foam insulation on the basement wall, and then strapping, and then blue (mold and moisture resistant) gypsum drywall sheets.